1. Field of the Invention
This invention relates to polyphosphite ligands. In one aspect the invention relates to the manufacture of polyphosphite ligands while in another aspect, the invention relates to removing residual solvent from finished polyphosphite ligand product.
2. Description of the Related Art
The preparation of polyphosphite ligands is well known and is described in, among other references, U.S. Pat. Nos. 4,769,498 and 5,688,986, US 2007/0112219 and WO 93/03839. The condensation reactions of these processes are typically performed in an inert hydrocarbon solvent, e.g., toluene, and produce polyphosphite crystals. The solvent is removed from the crystals by any convenient means, e.g., distillation, and then the crystals are dissolved in a second or wash solvent, e.g., ethyl acetate, to remove impurities. The polyphosphite is then re-crystallized from the second solvent and recovered by conventional means, e.g., centrifugation with the impurities remaining in the solvent.
Over the past 20 years or so, different manufacturing entities have experienced difficulties in the last step in the production of polyphosphite, i.e., the removal of the wash solvent from which the polyphosphite is re-crystallized. This step usually requires drying the product to less than (<) 0.5 weight percent (wt %) residual wash solvent. In some cases, extreme vacuum and elevated temperatures are required to dry the material and sometimes even 300 hours of drying is insufficient to meet specification. Residual wash solvent, even at these relatively low levels, can render the polyphosphite more vulnerable to degradation during storage and introduce unwanted variability in the final product. If the solvent is ethyl acetate, in some applications it can be considered a contaminate, particularly in pharmaceutical applications.
Without being bound by theory, centrifugation or high pressure filtration may compress the crude polyphosphite crystals and cause the formation of classical solvent of crystallization. Traditionally heating under vacuum is the conventional means to break a solvent of crystallization. However, this detail is not discussed in the art because laboratory testing of conditions and equipment used in the manufacture of polyphosphites does not exhibit solvent of crystallization. Strangely, this phenomenon appears to be limited to commercial operations and unless investigators are involved in scaling up a laboratory manufacturing process for polyphosphite production to a commercial process, they are not likely to observe the problem.